Predictable changes in aboveground allometry of trees along gradients of temperature, aridity and competition

Aim Trees are often observed to get shorter and more narrowly crowned in dry regions and at high elevations. We explore how this pattern is driven by two opposing factors: competition for light makes it advantageous to extend branches to their biomechanical limit, whereas under cold or arid conditions it is advantageous to have shorter branches, thereby reducing the length of the hydraulic transport system and embolism risk. Using data from 700,000 trees of 26 species, we quantify how environmental conditions influence the scaling of height and crown diameter (CD) with stem diameter (d.b.h.). We compare our predictions with those of metabolic scaling theory (MST), which suggests that allometry is invariant of environment. Location 48,000 inventory plots that systematically sample mainland Spain, a region in which climate varies strongly. Methods We fit d.b.h.height and d.b.h.CD functions using Bayesian methods, allowing comparison of within- and across-species trends in allometry along gradients of temperature, precipitation, drought and competition for light (i.e. the basal area of taller trees). Results The competitive environment had a strong influence on aboveground allometry, but all trees were far shorter than predicted by biomechanical models, suggesting that factors other than biomechanics are important. Species that dominate in arid and cold habitats were much shorter (for a given diameter) than those from benign conditions; but within-species heights did not vary strongly across climatic gradients. Main conclusions Our results do not support the MST prediction that d.b.h.height and d.b.h.CD allometries are invariant, or that biomechanical constraints determine height allometry. Rather, we highlight the role of hydraulic limitations in this region. The fact that intra-specific adjustment in d.b.h.CD height allometry along environmental gradients was far weaker than across-species changes may indicate genetic constraints on allometry which might contribute to niche differentiation among species.